In steelmaking process equipment, alumina-spinel based, alumina-magnesia based and alumina-spinel-magnesia based unshaped refractory materials as disclosed in the following Non-Patent Documents 1 and 2 are used as materials for a refractory construction to be subjected to a contact with molten steel and/or slag. As used in this specification, the term “spinel” means an MgO—Al2O3 based compound, more specifically, a compound having a stoichiometric composition of MgO.Al2O3, or a compound having a non-stoichiometric composition, in which MgO or Al2O3 is excessively incorporated as a solid solution.
The alumina-spinel based unshaped refractory material has a feature of providing high corrosion resistance against oxygen cleaning and steelmaking slag and high slag infiltration resistance, because spinel is capable of incorporating FeO therein as a solid solution, and further providing high hot strength, and high volume stability because of small residual expansion/shrinkage after heat receiving.
As for the alumina-magnesia based unshaped refractory material, when a refractory construction formed thereof is heated to 1100 to 1200° C. or more during steelmaking process, alumina and magnesia in the material read with each other to form fine spinel particles, and the fine spinel particles provide high corrosion resistance and slag infiltration resistance. Because finer spinel particles provide higher corrosion resistance and slag infiltration resistance, the alumina-magnesia based unshaped refractory material is generally better in terms of corrosion resistance and slag infiltration resistance, as compared, to the alumina-spinel based unshaped refractory material. However, the reaction causing the formation of fine spinel particles involves volume expansion, wherein expansion along with the formation of fine spinel particles occurs only on the side of an operating, surface of the refractory construction which is subjected to heating, thereby leading to a disadvantage that large peeling or crack occurs on the side of the operating surface. Thus, in terms of thermal spalling resistance, the alumina-magnesia based unshaped refractory material is generally interior to the alumina-spinel based unshaped refractory material.
The alumina-spinel-magnesia based unshaped refractory material has both features of the alumina-spinel based and alumina-magnesia based unshaped refractory materials, and, in terms of volume stability, corrosion resistance and slag infiltration resistance, exhibits intermediate characteristics between the two unshaped refractory materials.
The following Patent Document 1 discloses an alumina-spinel based unshaped refractory material using fine spinel particles having a particle size of 5 μm or less. This is intended to improve corrosion resistance and slag infiltration resistance of a refractory construction formed of the alumina-spinel based unshaped refractory material, by initially incorporating thereinto fine spinel particles which are equivalent to fine spinel particles to be formed in a refractory construction formed of the alumina-magnesia based unshaped refractory material during steelmaking process.
Each of the Non-Patent Documents 1 and 2 and the following Patent Document 2 discloses an alumina-spinel based unshaped refractory material and an aluminum-spinel-magnesia based unshaped refractory material each using alumina cement containing spinel particles (hereinafter referred to as “spinel-containing alumina cement”).
A commonly-used type of alumina cement comprises, as a primary component, CaO.Al2O3 or CaO.2Al2O3, and some types of alumina cements additionally contain 12CaO.7Al2O3, Al2O3, etc. Each of CaO.Al2O3, CaO.2Al2O3 and 12CaO.7Al2O3 exhibits a hydraulic properly, and is therefore used as a binder for an unshaped refractory material.
In an alumina-spinel based or alumina-spinel-magnesia based unshaped refractory material using such a commonly-used type of alumina cement, a portion of a refractory construction formed thereof adjacent to its operating surface during steelmaking process has a phase consisting primarily of Al2O3, spinel and CaO.6Al2O3, wherein a region other than the Al2O3 as an aggregate, called “matrix”, is formed primarily of spinel and CaO.6Al2O3. When these two phases are finely and uniformly dispersed over the matrix, the refractory construction can exhibit high corrosion resistance and slag infiltration resistance.
The spinel-containing alumina cement disclosed in the Non-Patent Documents 1 and 2 is a cement over which fine spinel particles and CaO.Al2O3 and CaO.2Al2O3 particles are uniformly dispersed. Further, a refractory construction, formed of the alumina-spinel based or alumina-spinel-magnesia based unshaped refractory material using the spinel-containing alumina cement can exhibit high corrosion resistance and slag infiltration resistance, because spinel and CaO.6Al2O3 are formed in a matrix in a finely and uniformly dispersed manner.